1. Field of the Invention
The present invention relates to the field of development of an underground reservoir such as a hydrocarbon reservoir. In particular, the invention concerns the geometrical characterization of such reservoirs.
The petroleum industry, and more precisely petroleum reservoir exploration and development, requires knowledge of the underground as perfect as possible to efficiently provide evaluation of reserves, production modelling or development management. In fact, determining the location of a production well or of an injection well, the necessary parameters for optimum hydrocarbon recovery, such as injection pressure, drilling mud composition, completion characteristics, require good knowledge of the reservoir geometry.
The petroleum industry has combined for a long time technical measurements with modelling performed in the laboratory and/or by softwares. Petroleum reservoir modelling thus is an essential technical stage of reservoir exploration or development. The goal of modelling is to provide a description of the reservoir via its geometry, its sedimentary architecture or its petrophysical properties.
2. Description of the Prior Art
A well-known oil or gas prospecting technique carries out seismic prospecting surveys of the subsoil. Geophysicists often use seismic reflection techniques to image the subsoil structure.
These techniques emit acoustic signals at the surface and record them after their successive reflections on the interfaces between geologic layers referred to as geologic discontinuities.
The accuracy of these seismic images of the subsoil obtained from seismic reflection surveys is of fundamental importance because it is from the analysis of the images that the geometry and the volume of the reservoirs, that, for example, the position and the geometry of new wells to be drilled, etc., are determined.
These imaging methods are based on the measurement of the traveltimes of the waves reflected on the main discontinuities of the subsoil. These traveltimes are the kinematic information that is used to determine the velocities of propagation of the seismic waves in the layers forming the subsoil. From these propagation velocities, the seismic records (measurements as a function of time) are converted to a depth image of the subsoil. This is referred to as time/depth conversion.
However, several representations of the subsoil velocities can explain the kinematics of seismic data. External information on the expected velocity representation is then necessary to remove the indeterminacy. This information can be data recorded in wells, but it must then be possible to determine the subsoil velocities while accounting for this type of information, in addition to the seismic information.
Determination of the subsoil velocity model is generally performed at the start of the petroleum exploration chain. This is carried out with little or no additional information. In fact, few wells have generally been drilled at this stage. Thus, quite often, the seismic subsoil image obtained in this context shows seismic reflectors that do not perfectly adjust with the seismic reflector depths recorded in wells.
A structural calibration of the seismic image with the well data is then essential prior to reservoir surveys. This calibration is then generally performed a posteriori. It updates the pre-existing velocity model. This velocity model modification is most often carried out via rudimentary methods that update the velocity model only locally, vertically to the well, without taking into account the possible wave propagation complexity in the case of horizontal plane non-homogeneous media or the wavelengths of the velocity variations contained in the initial velocity model.
Thus, the most commonly used method for adjusting a velocity model to the information recorded in wells produces, from the difference between the depth of a seismic marker observed in wells and that of the corresponding marker in a seismic depth image, a multiplicative or additive corrective coefficient that is applied to the velocity column located above the measuring point being considered and generally up to the depth of the previously calibrated upper seismic marker. If several wells are considered simultaneously, an interpolation between wells of the corrective values can be performed and the result is applied to the entire velocity model being considered. This technique is based on very strong medium simplifying hypotheses, poorly representative of the majority of the fields to be developed which the medium is made up of horizontal plane layers and the velocities do not vary much laterally. Because it is fast, easy to implement and/or for lack of other methods, this technique is however commonly used beyond its limited range of application. An erroneous image is then produced, which leads to erroneous interpretations and conclusions when characterizing a reservoir to define its conditions.
There is thus no satisfactory method in practice for using data newly acquired during the development of a reservoir, so as to acquire more precise knowledge of the underground geology and thus to provide a better evaluation of the reserves production modelling or development management.